The trend to continue to miniaturize semiconductor integrated circuits to achieve submicron feature sizes and increase the number of devices fabricated on the integrated circuit has required smaller isolation areas between devices. The active areas in which devices are built are isolated by a layer of oxide known as field oxide. However, the technology used to isolate active areas has not kept pace with the ever-decreasing device geometries.
Isolation techniques should meet a variety of requirements. First, active areas should be in close proximity. Second, the lateral encroachment or tapering of the field oxide into the active areas, known as "birdbeaking", should be minimized. Third, the leakage current between active devices should be negligible. Fourth, the process for forming the field oxide regions must be easily adapted for use with standard integrated circuit fabrication process flows and not adversely affect device parameters.
Many methods have been proposed over the years to reduce the bird's beak of a field oxide region when isolating devices. One such method of isolating devices, LOCOS, local oxidation of silicon, produces regions of insulating silicon dioxide between devices. The LOCOS process was a great technological improvement in reducing the area needed for the isolation regions and decreasing some parasitic capacitances.
In LOCOS, silicon nitride is deposited and patterned over a stress relief pad oxide layer. The silicon nitride layer is retained over the area over where further oxidation is not desired. Thus, the silicon nitride is etched to expose a portion of the pad oxide where the field oxide is to be grown. After the thermal oxidation of the exposed pad oxide to form the field oxide regions, the silicon nitride layer is removed.
Several problems occurred, however, with LOCOS. Thermal oxidation in the original LOCOS form always incurred lateral encroachment, or birdbeaking, of the field oxide into the active areas growing under the silicon nitride mask. This birdbeaking is a substantial sacrifice of active areas that becomes significant for feature sizes less than 1.5 microns. The active area becomes smaller than the initial dimensions of the nitride layer.
Attempts to suppress birdbeaking in LOCOS, such as forming thicker nitride layers, caused stress-related defects in the nearby substrate due to the difference in the thermal coefficients of expansion between the silicon substrate and the silicon nitride layers. Process complexity also increased substantially in attempting to avoid these stress-related defects. To achieve submicron geometries, there can be little or no physical loss of the active areas as occurs with the birdbeaking phenomenon.
To reduce the bird's beak effect, there has been proposed the use of a polysilicon layer between the nitride layer and the pad oxide layer as more fully described in U.S. Pat. No. 4,407,696, issued Oct. 4, 1983 to Han et al. The use of the polysilicon layer in the LOCOS process, known as poly-buffered LOCOS or PBLOCOS, is used to reduce oxidation induced stacking faults resulting from the stress caused by the different thermal coefficients of expansion between the silicon substrate and a thick silicon nitride layer overlying the substrate. As described more fully in the publication "Twin-White-Ribbon Effect and Pit Formation Mechanism in PBLOCOS", J. Electrochem. Soc., Vol 138, No. 7, Jul. 1991 by Tin-hwang Lin et al, the polysilicon layer absorbs the excessive stress caused by the silicon nitride and prevents the lateral encroachment of oxidants, thus reducing the bird's beak.
The field oxide layer grown using poly-buffered LOCOS thus comprises the oxide derived from the silicon substrate, a portion of the pad oxide layer and oxide derived from the polysilicon layer. Afterwards, the nitride layer, the polysilicon layer and the pad oxide are etched. The poly-buffered LOCOS process reduces the bird's beak area over standard LOCOS resulting in less encroachment of the tapered portion of the field oxide into the active areas under the nitride mask. However, the bird's beak effect still remains, due to the oxidation of the polysilicon layer.
In order to further decrease the bird's beak area using poly-buffered LOCOS, it has been proposed in an article by J. M. Sung et al, titled "Reverse L-Shape Sealed Poly-Buffer LOCOS Technology,"in IEEE, 1990, pages 549-551, to form the pad oxide over the substrate as in the conventional process. A polysilicon layer is formed over the oxide and a silicon nitride layer is formed over the polysilicon. The nitride/polysilicon stack is etched down to the pad oxide. The pad oxide is then selectively etched resulting in lateral undercuts beneath the polysilicon. A new buffer oxide layer is then grown. A nitride spacer is then formed along the sides of the polysilicon and in the area of the lateral undercut. The article indicates that the bird's beak is thus reduced when the field oxide is grown in the active areas because, among other reasons, the polysilicon cannot be oxidized.
The nitride sidewall will reduce the bird's beak but does not prevent the thick nitride layer on top of the polysilicon layer from inducing defects in the silicon substrate. During the formation of the field oxide region in a steam ambient, a chemical reaction takes place in which water molecules react with silicon nitride to form oxide and ammonia. Nitridation occurs in the polysilicon layer due to the ammonia reacting with the polysilicon. The chemical reaction forms silicon nitride pits or defects in the polysilicon. Then, when the nitride layer above the polysilicon is etched away, these silicon nitride pits within the polysilicon are also etched away causing defects or pits in the underlying oxide layer. When the polysilicon is subsequently etched away, the etch step proceeds through the defects in the oxide and are transferred to the substrate.
If a nitride layer is formed between the polysilicon layer and the oxide layer, the pit formations in the polysilicon layer cannot be transferred to the underlying oxide layer. It would be desirable to reduce the bird's beak while also reducing the defects in the oxide layer and the substrate. It would further be desirable to reduce the complexity of the process and be easily adapted for use with standard integrated circuit fabrication process flows.